Method and apparatus for maintaining a preselected partial pressure

ABSTRACT

Improving gas-mixing methods and apparatus for maintaining a preselected oxygen partial pressure in breathing gas supplied to a diver under an abnormal pressure. A mixing tank with separate gas inputs is provided to receive and blend together oxygen and filler gas according to a preselected ratio as breathing gas is withdrawn from the tank by the diver. More particularly, a pair of helical tubing lengths or strings are arranged about the inside surface of the tank to receive and dispense filler gas and oxygen uniformly along the length of the tank. A control circuit is also provided to continually measure the molecular oxygen content of the tank, and to admit oxygen whenever the molecular oxygen content drops below a preselected level.

United States Patent [72] Inventor Max A. W. Reiher 2,915,059 12/1959 LeMasson 137/88 Gretna, La. 3,185,448 5/1965 Fraser et a1. 259/4 [21]AppLNo. 757,317 3,215,057 11/1965 Turek 98/115 ii :"f t d i i g PrimaryExaminer-Rober1 G. Nilson i c i E l Attarneys-Arnold, Roylance, Krugerand Durkee, Tom 1 1 i Arnold, Donald c. Roylance, Walter Kruger, BillDurkee,

Frank S. Vaden, 111 and Edmund F. Bard [54] METHOD AND APPARATUS FORMAINTAINING A PRESELEC'I'ED PAR-"AL PRESSURE ABSTRACT: improvinggas-mixing methods and apparatus 10 Claims, 2 Drawing Figs formaintaining a preselected oxygen partial pressure 111 breathing gassupplied to a diver under an abnormal pressure. [52] US. Cl 137/88, A ii t nk with eparate gas inputs :is provided to receive 98/ 1.5, 128/142,128/204, 137/604, 259/4 and blend together oxygen and filler gasaccording to a Int. preselected ratio as breathing gas is withdrawn fromthe tank 605d 1 1/035 by the diver. More particularly, a pair of helicaltubing lengths of 88, or trings are arranged about the inside surface ofthe tank to 93, 604; 98/] .5; 128/142, 140, 142.3, 204; 2 receive anddispense filler gas and oxygen uniformly along the I length of the tank.A control circuit is also provided to con- [56] References cued tinuallymeasure the molecular oxygen content of the tank, UNITED STATES PATENTSand to admit oxygen whenever the molecular oxygen content 2.830.5834/1958 Finney, Jr 128/142 drops below a preselected level.

corvmor i UNIT HE L IUM SUPPL Y OXYGEN SUPPLY METHOD AND APPARATUS FORMAINTAINING A PRESELECTEI) PARTIAL PRESSURE BACKGROUND OF INVENTION Thisinvention relates to improved methods and apparatus for blending ormixing gases, and more particularly relates to methods and apparatus formixing breathing gases according to a predetermined formulation duringthe supply of such gases to an artificial breathing atmosphere such asthat provided for divers.

It is well known that the normal breathing atmosphere for humans andother nonaquatic animals is a mechanical blend or mixture of gases, andthat this mixture is normally composed of about percent oxygen to 80percent nonsustaining gas (mostly nitrogen). It is also well known thatit is harmful for life which depends on gaseous oxygen to be subjectedfor any appreciable period to a breathing atmosphere which containseither too much or too little oxygen. However, what is not generallyknown is that it is not the percentage of oxygen in a breathingatmosphere which is important from the standpoint of life support.Instead, it is the molecular amount of free gaseous oxygen which must bepresent for life-sustaining purposes.

It is well known that divers, aviators, etc., are often required tooperate for extended periods under extremely abnormal pressures, and inan ambient atmosphere which requires that they be continuously suppliedwith a flow of breathable gas. In the case of divers, for example, it isnot uncommon for them to be subjected to ambient pressures which aremany times greater than normal atmospheric pressure, and thus breathinggas must be supplied at the same abnormal pressure. In the case ofaviators, the pressure is almost always only a fraction of normalatmospheric pressure. Nevertheless, in both instances the artificialatmosphere must be composed of the same molecular amount of oxygen asthat normally required for life support purposes. Thus, the percentageof oxygen in the artificially supplied breathing gas must be variedaccording to the pressure of the breathing gas supplied.

For example, each 100 molecules of gas in a normal breathing atmospheremay be assumed to include 20 molecules of free oxygen and 80 moleculesof nitrogen and other free gases. If the diver is submerged to a depthsuch that his ambient pressure is doubled, the pressure under which hisbreathing gas is supplied must also be doubled. However, if the mixturewhich is proper at a pressure of I atmosphere is merely supplied at apressure of 2 atmospheres, it will be apparent that the diver willreceive a breathing mixture which is actually harmful because itcontains twice as many oxygen molecules. Instead, what is required isthat the 200 gas molecules present in a cubic unit of breathing gassupplied under a pressure of 2 atmospheres, must be composed of only theoriginal 20 molecules of oxygen with the other 180 molecules being aninert or relatively inert gas such as nitrogen or helium.

It is rather difficult to mix oxygen and helium or nitrogen forbreathing purposes according to precise percentages, and most of themixing apparatus and methods of the prior art involve rathertime-consuming techniques. Furthermore, no satisfactory technique hasever been developed whereby the breathing gas mixture may be selectivelyvaried as it flows to the diver. These disadvantages of the prior art,however, are overcome with the present invention, and improved methodsand apparatus are provided for selectively varying the partial pressureof the oxygen in a breathing gas supply during the use of such supply bya diver or other utilization system.

SUMMARY OF INVENTION In apreferred embodiment of the present invention,a cylindrical closed mixing tank is provided which includes an oxygenintake port at one end and a helium-(or nitrogen) intake port at theother opposite end. Oxygen enters the mixing tank through a helicallength of tubing which is arranged along the length of tank and spirallyabout its inside surface. One end of the tubing is closed, and the otherend is connected to the oxygen intake port. A plurality of spaced-apartports are provided in the wall of the tubing facing always toward thelongitudinal axis of the mixing tank. These ports may all be the samesize. However, it is desirable that they be progressively larger as theyapproach the closed farthest end of the tubing, and the size and spacingof the ports should be such that the sum of the areas of all of theports not be greater than the inside cross-sectional area of the tubing.

A second similar helical length of tubing is also similarly arrangedabout the inside surface of the mixing tank, with its open end connectedto the helium intake port. Helium is received under pressure from aseparate helium supply, and continually flows into the tank except whenthe pressure in the mixing tank is equal to or greater than the pressureof the helium supply. Accordingly, a check valve is preferably includedto block backflow of gas from the mixing tank into the helium supply.

A separate supply of oxygen, preferably under a constant pressuresubstantially greater than the maximum expected pressure in the mixingtank, is connected to supply oxygen to the mixing tank by way of thefirst helical length of tubing. A suitable control system is alsoprovided to continually measure the partial pressure of the oxygen inthe mixing tank, and to generate a functionally related electricalsignal whenever the partial pressure of the oxygen is less than apreselected partial pressure. A solenoid-controlled valve is preferablyinterconnected between the oxygen supply and the oxygen intake port,which opens in response to the electrical signal, and which snaps shutwhen the signal is discontinued. Thus, tiny jets of oxygen aremomentarily injected into the mixing tank along its interior each timethe partial pressure: of the oxygen drops below the preselected partialpressure sought to be maintained in the mixing tank and as the breathing:mixture is drawn from the mixing tank for the diver.

It is well known that helium cannot be effectively intermixed withoxygen if it is put in on top" of the oxygen, unless mechanical fans orstirring mechanisms are thereafter used to achieve intermixing of thegases. For this reason, it is common practice to vary the mixture byadding oxygen rather than by adding helium.

This disadvantage of the prior art has been eliminated by the methodsand apparatus of the present invention, since the turbulence created bythe jets of helium emitted from the helical tubing will intermix theinput helium throughout the mixing tank. Accordingly, when it is desiredto reduce the partial pressure of the oxygen in the breathing gasbeingsupplied to a diver, the control system may simply be adjusted tocall for a reduction of oxygen in the mixing tank, whereupon pure heliumwill be injected into the mixing tank to replace withdrawals ofbreathing gas by the diver. In this manner, the diver will simply breathoff the excess oxygen until the proper ratio is achieved.

The foregoing technique is, of course, somewhat time consuming. If amore rapid reduction in oxygen partial pressure must be achieved, thecontrol system may be adjusted to select the new ratio sought to beachieved, and the mixing tank may thereafter be vented until themolecular oxygen content of the tank drops below this new ratio. Afterthis, the vent may be closed and the control system may then bepermitted to inject helium and oxygen in the preferred manner until theproper balance and pressure is attained in the mixing tank.

Accordingly, it is an object of the present invention to provide novelmethods and apparatus for supplying a preselected partial pressure ofbreathing oxygen to a life support system.

It is further an object of the present invention to provide improvedmethods and apparatus for selectively controlling the partial pressureof the oxygen in a subsea life support system and the like.

It is also an object of the present invention to provide improvedmethods and apparatus for varying the partial pressure of oxygen in asealed atmosphere of breathing gas.

It is further an object of the present invention to provide methods andapparatus for maintaining a preselected partial pressure of the oxygenin a sealed breathing atmosphere under a varying pressure.

As hereinafter stated, it is well known that it is difficult and I timeconsuming to establish a uniform mixture of oxygen and helium ornitrogen without the use of mechanical whipping or stirring mechanisms.Thus the effectiveness of the present invention is dependent to asubstantial degree on the use of the gas streams emanating from theports in the helical tubing coils to achieve turbulence in the gasmixture within the mixing tank. Furthermore, it is a particularadvantage that the mixing of the breathing atmosphere be performedduring the withdrawal of breathing gas from the mixing tank by the diveror other utilization system. Accordingly, it will be apparent that it isdesirable that the turbulence created in the mixing tank by the gas jetsemanating from the ports in the helical tubing lengths, be establishedin a relatively uniform manner throughout substantially the entirelength of the mixing tank, and that there be a uniform oxygen partialpressure throughout the mixing tank at all times.

Accordingly, it is a feature of the present invention that both helicaltubing lengths be arranged in the mixing tank concentrically about thelongitudinal axis of the tank, and that both such tubing lengths extendwithin the tank along substantially its entire length.

It is a further feature of the present invention that the ports in eachhelical tubing length face the longitudinal axis of the mixing tank, andthat such ports be progressively larger in diameter, from the intakeport to which the tubing is connected to the closed end of the tubinglength, whereby each gas component will issue into the mixing tank at auniform rate along the length of the tank.

It is further a feature of the present invention that the helium andoxygen intake ports be located at opposite ends of the mixing tank.

It is another feature of the present invention that the sum of the areasof the ports in each helical tubing length not exceed the total insidecross-sectional area of the tubing.

These and other objects, features and advantages of the presentinvention will be apparent from a consideration of the followingdetailed description, wherein reference is made to the figures in theaccompanying drawing.

IN THE DRAWING FIG. 1 is a functional representation of an exemplaryembodiment of a mixing system incorporating features of the presentinvention and suitable for use in supplying breathing gas to a deep seadiver or the like.

FIG. 2 is a pictorial representation, partly in cross section, of anexemplary mixing tank suitable for use with the mixing systemillustrated in FIG. ll.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen afunctional representation of a helium supply 30, such as a conventionalsteel bottle or the like, and a functional representation of a similaroxygen supply 6. inasmuch as the depicted system is primarily intendedto supply a breathing gas suitable for human life support, it will beapparent that the oxygen supply 6 contain relatively pure oxygen only.The helium supply 30 may contain either nitrogen or helium, depending onthe pressure at which the breathing gas is expected to be utilized, butin either case, it is preferable that the filler gas be relatively pure.

As may further be seen, a mixing tank 2 is provided which includes asuitable drain pipe 3 and valve 4, and which further includes provisionfor receiving oxygen and helium (or nitrogen) at opposite ends. Thehelium flows from the helium supply 30 to the mixing tank 2, by way of ahelium input circuit 41 including a helium supply shutoff valve 31, avariable helium pressure regulator 32 for reducing the pressure of thehelium, a check valve as for preventing backflow from the mixing tank 2,and a helium intake shutoff valve 35. An emergency helium bypass valve33 may also be provided in case of failure of the helium pressureregulator 32 to pass helium into the check valve 34L Oxygen may be seento flow from the oxygen supply 6 to the mixing tank 2, by way of anoxygen input circuit 42 including an oxygen supply shutoff valve 8, avariable oxygen pressure regulator 10 for reducing the pressure of theoxygen, a normally closed valve 12 actuated by a solenoid 14, and anoxygen intake shutoff valve 16. A bypass valve 9 may be included tobypass oxygen around the oxygen pressure regulator 50, and a similarbypass valve 15 may be included to bypass oxygen around thesolenoid-controlled valve 12.

Breathing gas may be seen to be taken from the mixing tank 2 by way of aconventional pneumatic hose 17, or the like, which may be connected to apoint of utilization such as a diver or a pressure bottle or tank soughtto be charged, and which preferably includes a suitable outlet shutoffvalve 18. Breathing gas pressure in the mixing tank 2 may also bereduced by means of a vent line 19 having a suitable vent shutoff valve20.

In the depicted system, filler gas (helium or nitrogen or the like) isexpected to flow into the mixing tank 2 each time the breathing gaspressure in the tank 2 drops below the preselected operating pressure inthe tank 2. This operating pressure is necessarily always higher thanthe environmental or ambient pressure surrounding the diver, since it isnecessary to drive breathing gas to the diver through a long pneumatichose 17 or the like. For deeper dives, the operating pressure in themixing tank 2 will usually be 50l00 p.s.i.g. higher than theenvironmental pressure, and often even higher.

In the system depicted in FlG. 1, the operating pressure for the mixingtank 2 is selected and established by means of the variable helium orfiller gas pressure regulator 32. A suitable pressure gauge 23 with anassociated shutoff valve 24 may be included to afford means forcontinuous observation of the pressure within the mixing tank 2.

Helium or nitrogen input is used primarily as a filler gas to maintainthe desired pressure of the breathing gas in the mixing tank 2, and thusno input control is required for the filler gas other than the regulator32. Oxygen input must be limited to that necessary to maintain thepartial pressure of the oxygen in the mixing tank 2 according to lifesupport requirements as hereinbefore explained. Accordingly, a suitablecontrol system is preferably included, as hereinbefore stated, to openthe solenoid-controlled valve 12 whenever the partial pressure of theoxygen in the mixing tank 2 drops below the level sought to bemaintained, and to permit the valve 12 to close whenever the partialpressure of the oxygen rises above the level or valve sought to bemaintained. More particularly, a sensor 36 such as that described in US.Pat. No. 3,071,530, may be disposed in the interior of the mixing tank2, to continually generate an electrical signal functionally related inmagnitude to the partial pressure of the oxygen in the mixing tank 2.The sensor 36 is connected by a lead 37 to a suitable control circuit38, such as that presently manufactured by Teledyne, lnc. The controlcircuit 38 preferably receives the sensor output signal and continuallycompares such sensor signal with a selectively variable reference signalwhich is functionally related in magnitude to whatever partial pressuremay be sought to be maintained in the mixing tank 2. If the sensorsignal is equal to or greater than the selected reference signal, nocontrol output signal will be generated and the solenoid-controlledvalve 112 will remain closed, whereby helium or nitrogen will flow intothe mixing tank 2 to replace breathing gas exiting through the hose 17until the partial pressure of the oxygen in the mixing tank 2 is reducedto less than the partial pressure sought to be maintained. When thisoccurs, however, the sensor signal will be less than the selectedreference signal, and the control unit 38 will respond to this unbalanceby generating or passing a suitable control signal (such as 1 15 volts,60 cycles AC) to the solenoid 14 by way of conductor 39. This will openthe solenoid-controlled valve 12, and oxygen from the oxygen pressureregulator will be injected into the breathing gas in the mixing tank 2.Since it is desirable that this injected oxygen be fully blended andintermixed throughout the interior of the mixing tank 2 within theshortest possible time interval, it is desirable that this injection ofoxygen create substantial atmospheric turbulence whereby suchintermixing may be adequately achieved. Accordingly, it is desirablethat the oxygen pressure regulator 10 be set to establish an oxygeninput pressure substantially greater (suchas 50 p.s.i.g.), than thepressure setting of the helium pressure regulator 32.

Although a reliable control unit 38 is available from several commercialsources, and although a sensor 36 such as that depicted in U.S. Pat. No.3,071,530 is generally quite dependable and accurate, it should beremembered that the system depicted in FIG. 1 is intended to providelife support under extremely dangerous conditions. Accordingly, it isdesirable to continually monitor the actual oxygen molecular content ofthe breathing gas in the mixing tank 2, and this may conveniently beachieved by an analyzer 40 such as that depicted in US. Pat. No.2,913,386. Such an analyzer 40 may be conveniently connected to samplethe contents of the mixing tank 2 by means of a suitable pipe orpneumatic hose 2] connected to the mixing tank 2 by a suitableshutoffvalve 22.

Referring now to H6. 2, there may be seen a more detailed illustrationof a mixing tank 102 suitable for use inthe system depicted functionallyin FIG. I. As represented, the tank 102 may be a closed, generallycylindrical vessel having a drain pipe 103 and drain shutoff valve 104,a vent pipe 119 and vent shutoff valve 120, and a pressure gauge 123with suitable shutoff valve 124, as hereinbefore explained. Breathinggas may be withdrawn by the diver by means of a conventional pneumatichose 117 having a suitable shutoff valve 118 as depicted, and thebreathing atmosphere in the tank 102 may be continually sampled by asuitable analyzer (not depicted) which may be connected to the tank 102by a pneumatic hose 121 and suitable shutoff valve 122. A sensor 136,such as that depicted in US. Pat. No. 3,071,530, may be suitablydisposed in the tank 102 to generate an electrical signal functionallyindicative of the molecular content of free oxygen in the tank 102, andsuch signal may e transmitted by a suitable lead 137 to a control unit(not depicted).

As hereinbefore explained, is the function of the mixing tank 102 toprovide a collection or blending region for oxygen and any suitablefiller gas such as helium or nitrogen. Accordingly, helium may bereceived through a helical tubing length 141 arranged in the tank 102 ina spiral manner adjacent the inside wall of the tank 102 and about itslongitudinal axis. The helical tubing length 1141 has a closed end, andthus the helium or other filler gas emanates into the tank 102 throughparts 150 which are spaced along the tubing 141 inside the tank 102 anduniformly facing the longitudinal axis of the tank 102. These ports 150may be any suitable size and spacing, but it is desirable that the ports150 increase in size progressively along the length of the tubing 141toward its closed end. Further, it is especially desirable that the sumof the areasof the ports 150 not exceed the inside cross-sectional areaof the tubing Ml.

As may be see in FIG. 2, oxygen may be delivered into the tank 102through another similar helical tubing length 142 having one closed endwithin the tank 102 and having its other open end connected to receiveoxygen as hereinbefore explained. Accordingly, a series of ports 151facing the longitudinal axis of the tank 102 may. be provided in asuitable spaced-apart arrangement along the length of the helical tubing142, and these ports may progressively increase in areal size toward theclosed end of the tubing 142. In addition, it is particularly desirablethatthe sum of the areas of these ports 151 not exceed the insidecross-sectional area of the tubing 142.

Although the use of the presentinvention for the purpose of subsea lifesupport has been emphasized herein, it should be clearly understood thatthe invention may also be used to provide a properly oxygen-enrichedbreathing mixture for aviators and the like. When the invention isemployed for this alternate purpose, the operating pressure inthe mixingtank 2.

must be kept at a level greater than 1 atmosphere notwithstanding thatthe environmental pressure of the aviator will be less than 1 That andthat the oxygen ratio of the breathing gas is greater than the ratiowhich is normally life sustaining at a l atmosphere pressure.

Another feature of the present invention involves the size and shape ofthe mixing tank 2 depicted in FIG. ll. As hereinbefore stated, it isespecially suitable that the tank 2 have a generally cylindrical shareas illustrated by thy mixing tank 102 depicted in MG. 2. The tank 2 mayalso be spherical in shape, and other suitable configurations maysuggest themselves from a consideration of the principles of the presentinvention. hat is more important is that the size of the mixing tank T02is functionally related to the number and size of the ports 150 andll5ll in the two helical tubings M1 and 142. As hereinbefore explained,the size and number of these ports 1150 and 151 determine the amount ofturbulence which intermixes the helium with the other breathing gases inthe tank 102. If the tank 102 is too large the turbulence may beinadequate, and thus the diameter of the tank 102 is directlyfunctionally related to the number and size of the ports T50 and i.

Many other modifications and variations in the structures depicted anddescribed herein will be readily apparent from a consideration of theconcept of the present invention. Accordingly, it should be clearlyunderstood that the methods and apparatus described herein and depictedin the accompanying drawing, are illustrative only and are not intendedas limits to the scope of the invention.

What I claim is:

l. A mixing system for establishing and maintaining a gas mixture havinga preselected oxygen partial pressure which comprises:

a first gas supply for providing a filler gas under a first preselectedpressure;

an oxygen gas supply for providing oxygen under a second preselectedpressure;

a mixing tank having a filler gas intake port connected to said firstgas supply and an oxygen intake port connected to said oxygen gassupply;

first tubing means communicating with said filler gas intake port andspirally arranged proximate the inner surface of said mixing tank;

second tubing means communicating with said oxygen gas intake port andspirally arranged proximate the inner surface of said mixing tank;

said first and second tubing means having a plurality of sidewallopenings therein generally directed toward the interior of said mixingtank; and

control means for sensing the oxygen partial pressure in said mixingtank and admitting oxygen to said mixing tank in amounts to establishsaid oxygen partial pressure in said mixing tank at said preselectedoxygen partial pressure.

2. The system described in claim ll wherein said oxygen gas supplyprovides said oxygen at a second preselected pressure which issubstantially greater than said first preselected pressure.

3. The system described in claim 2 including first selectively variablepressure-regulating means for establishing said first preselectedpressure and to supply filler gas'to said mixing tank when the internalpressure of said tank is less than said first preselected pressure.

4. The system describe in claim 3 including a check valve interconnectedbetween said first selectively variable pressure regulator and saidmixing tank to prevent backflow of said gas from said mixing tank tosaid first gas supply.

5. The system described in claim ll including a normally closed controlvalve interconnected between said oxygen gas supply and said mixingtank, said valve being adapted to be opened by said control means.

6. The system described in claim wherein said oxygen gas supply includesa second selectively variable pressure regulator interconnected betweensaid oxygen supply and said control valve for establishing said secondpressure.

7. The system described in claim 1 wherein said mixing tank comprises agenerally cylindrical vessel; and where said first and second tubingmeans are arranged proximate the inner cylindrical surface of saidvessel along substantially the entire length of said vessel.

8. The system described in claim 7 wherein said first and second tubingmeans have said sidewall openings directed generally toward thelongitudinal axis of said cylindrical ves-

1. A mixing system for establishing and maintaining a gas mixture havinga preselected oxygen partial pressure which comprises: a first gassupply for providing a filler gas under a first preselected pressure; anoxygen gas supply for providing oxygen under a second preselectedpressure; a mixing tank having a filler gas intake port connected tosaid first gas supply and an oxygen intake port connected to said oxygengas supply; first tubing means communicating with said filler gas intakeport and spirally arranged proximate the inner surface of said mixingtank; second tubing means communicating with said oxygen gas intake portand spirally arranged proximate the inner surface of said mixing tank;said first and second tubing means having a plurality of sidewallopenings therein generally directed toward the interior of said mixingtank; and control means for sensing the oxygen partial pressure in saidmixing tank and admitting oxygen to said mixing tank in amounts toestablish said oxygen partial pressure in said mixing tank at saidpreselected oxygen partial pressure.
 2. The system described in claim 1wherein said oxygen gas supply provides said oxygen at a secondpreselected pressure which is substantially greater than said firstpreselected pressure.
 3. The system described in claim 2 including firstselectively variable pressure-regulating means for establishing saidfirst preselected pressure and to supply filler gas to said mixing tankwhen the internal pressure of said tank is less than said firstpreselected pressure.
 4. The system describe in claim 3 including acheck valve interconnected between said first selectively variablepressure regulator and said mixing tank to prevent backflow of said gasfrom said mixing tank to said first gas supply.
 5. The system describedin claim 1 including a normally closed control valve interconnectedbetween said oxygen gas supply and said mixing tank, said valve beingadapted to be opened by said control means.
 6. The system described inclaim 5 wherein said oxygen gas supply includes a second selectivelyvariable pressure regulator interconnected between said oxygen supplyand said control valve for establishing said second pressure.
 7. Thesystem described in claim 1 wherein said mixing tank comprises agenerally cylindrical vessel; and where said first and second tubingmeans are arranged proximate the inner cylindrical surface of saidvessel along substantially the entire length of said vessel.
 8. Thesystem described in claim 7 wherein said first and second tubing meanshave said sidewall openings directed generally toward the longitudinalaxis of said cylindrical vessel.
 9. The system described in claim 1wherein the inside cross-sectional area of said first tubing means andof said second tubing means is in each case greater than the sum of theareas of the sidewall openings in each of said first and second tubingmeans respectively.
 10. The system described in claim 1 wherein saidsidewall openings in said first tubing means and said second tubingmeans increase in size with increasing distance of said openings fromthe intake port to which said tubing means is connected.